A capacitive micromachined ultrasonic transducer (CMUT) device can be used to either transmit or receive ultrasonic acoustic signals. Signal transmission is accomplished by electrostatically deflecting the CMUT membrane and thereby storing elastic energy in the membrane. Upon release, this elastic energy is converted to kinetic energy and produces an acoustic pulse. Signal reception, on the other hand, occurs when the CMUT membrane is deflected by an acoustic pulse, causing the gap between the upper and lower electrodes to change. Both transmit and receive operations require a controlled electrical quantity, either charge or voltage, to be placed on the CMUT electrode pair. In transmit mode, the electrical signal applied to a CMUT typically consists of an RF (radio frequency) voltage waveform added to a constant DC (direct current) baseline voltage. In receive mode, only a DC voltage is typically applied to the CMUT.
In receive mode, a DC voltage applied between the upper and lower electrodes of a CMUT is important for several reasons. First, the DC voltage source provides an electric field that causes opposite charges to be stored on the two CMUT electrodes. The capacitor formed by the CMUT electrodes obeys the relationship Q=C·V, where Q is the stored charge, C is the device capacitance and V is the applied voltage. According to this relationship, a higher DC voltage applied to the CMUT results in a larger quantity of charge initially stored on the CMUT. In receive mode, the gap between the electrodes changes in response to an impinging acoustic signal, causing the capacitance to change according to:
                    C        =                              ɛ            ·            A                                              d              0                        -                          d              ⁡                              (                t                )                                                                        (                  Eq          .                                          ⁢          1                )            where do is the initial gap between the electrodes, d(t) is the time-varying membrane displacement induced by the acoustic wave, A is the geometric area of the overlap between electrodes, and ∈ is the dielectric constant of the material between the electrodes. The output current from the CMUT corresponds to the first derivative of charge with respect to time. Because both the capacitance and voltage can vary with time, the expression for the output current of the CMUT becomes:
                    I        =                                            ⅆ              Q                                      ⅆ              t                                =                                    C              ·                                                ⅆ                  V                                                  ⅆ                  t                                                      +                          V              ·                                                ⅆ                  C                                                  ⅆ                  t                                                                                        (                  Eq          .                                          ⁢          2                )            In this expression, it becomes clear that a larger applied voltage V will result in a larger output current I, for a given acoustically induced capacitance change.
The second reason the DC voltage applied to the CMUT is important is that the DC voltage causes electrostatic attraction between the plates of the structure, causing the gap to be reduced as the DC voltage increases. According to Eq. 1, the capacitance of the CMUT increases as the gap is reduced, and, according to Eq. 2, the output current increases with increasing capacitance. The DC voltage on the CMUT reduces the gap until the point where the electrostatic attraction between the plates exceeds the restoring spring force of the membrane, causing the membrane to collapse. This voltage is known as the pull-in voltage, and is typically equal to the voltage required to reduce the gap by ⅓, as discussed in Y. Huang, A. Ergun, E. Haeggstrom, M. Badi, B. Khuri-Yakub, “Fabricating Capacitive Micromachined Ultrasonic Transducers With Wafer-Bonding Technology,” Journal of Microelectromechanical Systems, Vol. 12, No. 2, April 2003, which is incorporated in its entirety by this reference. CMUTs are typically biased at approximately 75 to 90% of the pull-in voltage, in order to take advantage of the reduced gap while maintaining a safe margin before membrane collapses. The benefits and detriments of increasing the bias voltage beyond the pull-in voltage and operating in the so-called “collapse” or “collapse-snapback” modes are discussed by B. Bayram, O. Oralkan, A. Ergun, E. Haeggstrom, G. Yaralioglu, B. Khuri-Yakub in “Capacitive Micromachined Ultrasonic Transducer Design for High Power Transmission,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, No. 2, February 2005, and US Published Application No. 2005/0234342, which are incorporated in their entirety by this reference.
The preceding discussion suggests that the DC voltage applied to a CMUT should be as large as possible to produce the maximum output signal. The range of acceptable DC voltage values is constrained, however, by the voltage limits of the given fabrication technology. A primary advantage of CMUTs as compared to piezoelectric ultrasound transducers is the possibility of integrating CMUTs on a common substrate with CMOS circuits such as amplifiers, switches, and analog-to-digital converters. Therefore, most CMUTs are manufactured using materials and fabrication processes that are, in principle, compatible with CMOS circuits. Standard CMOS circuit components such as transistors and capacitors are, however, damaged by voltages exceeding a few tens of volts. As shown in FIG. 1, in order to apply a large DC bias 112 to a CMUT device 110 without damaging the input transistors of the amplifier 124, a DC blocking capacitor 130 can be used, as indicated by Cblock 130. This configuration 100 is typical for CMUT readout circuits built using discrete (off-chip) components, since large value capacitors 130 with large voltage limits are available. By contrast, when implementing readout circuitry on chip, large capacitors require prohibitive amounts of circuit real estate, and these capacitors 130 are intended to be subjected only to circuit-compatible voltages (<30V).
Besides the challenges introduced by the integration of CMOS electronics on-chip with CMUTs, complications can arise due to trapped mobile charge in layers between CMUT plates, resulting from the constant application of a high-voltage bias to the CMUT. Since increasing the bias voltage ideally increases the sensitivity of a CMUT, these charging problems are exacerbated when aiming for high-sensitivity CMUT operation. One solution to this problem has been to change the CMUT structure, such that the surface area of dielectric layers between CMUT plates is reduced, thereby scaling back the amount of charge that can be trapped, as discussed in Y. Huang, E. Haeggstrom, X. Zhuang, A, Ergun, B. Khuri-Yakub, “A Solution to the Charging Problems in Capacitive Micromachined Ultrasonic Transducers,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 52, No. 4, April 2005 and US Published Application No. 2005/0228285, which are incorporated in their entirety by this reference. Depending on the precise structure of the CMUT, this solution can require additional photolithography steps and etching steps during the CMUT fabrication process, increasing the expense of the transducer device and reducing cost-savings provided by CMUT technology.
Thus, there is a need in the field of ultrasound imaging to create an improved system and method for reducing the effects of trapped charge in CMUTs. This invention provides such an improved system and method.